Electroplating of germanium



G. szEKELY ELECTROPLATING oF GERMANIUM Sept. 28, 1954 Filed Aug. 5. 1950/V/CIL 6477/005 mami/w aum GRAPH/7E ,4A/005 Patented Sept. 28, 1954 ITEDSTATES TENT OFFICE Gustav Szekely, New York, N. Y., assigner to Syl-Vania Electric Products Inc., a corporation of Massachusetts ApplicationAugust 5, 1950, Serial No. 177,885

14 Claims. l

This invention relates to the electrodeposition of germanium in the formof a bright metal'- lic coating,

Although germanium has been used for some time in the construction ofelectrical devices such as crystal rectifiers, transistors,photoelectric cells, etc., no satisfactory method of electrolyticallydepositing germanium had heretofore been devised. Although it is truethat germanium had been electrolyzed from ammoniacal ammonium tartratesolutions by the discoverer of the element it was reported to be apoorly adhering germanium film. Subsequently it was found that germaniumcould be electrodeposited in the form of a coherent steel gray lm fromaqueous potassium hydroxide solutions of germanium dioxide. After a thingermanium iilm had been electrodeposited by this method little if anyadditional germanium could be observed. Further electrolysis apparentlyyields mostly hydrogen. More recently Fink and Dokras have reported theuse of a molten salt bath and several organic baths for theelectrodeposition of germanium. The salt bath consists of a moltenmixture of borax and germanium dioxide with a graphite rod suspended inthe melt to serve as anode and a graphite crucible as cathode. Inaccordance with this method molten germanium could be collected at thebottom of the crucible. They also report the use of organic bathscontaining germanium iodide dissolved in glycerine, ethylene glycol ordiethylene glycol. Although it is possible to plate germanium from thelborax bath which had been described by the authors as a method ofelectrowinning the element and by doing so to obtain silvery adherentbut brittle lms of germanium metal, this method is unsatisfactory forthe following reasons: Firstly, its operating temperature range includesthe melting point of germanium, 958 C. At this temperature metals usedas cathodes readily diffuse or dissolve into the bath or into thedeposit thus leading to impure germanium. Secondly, as the cathode iswithdrawn from the bath a considerable amount of the melt adheres to itmaking it necessary to go through a lengthy operation of dissolving andleaching.

When the organic germanium iodide bath is used it is necessary tooperate the bath at temperatures in the range of 140 to 150 C'. At thistemperature a considerable decomposition takes place and considerablesolvent is lost by evaporation. Furthermore, a large quantity of othercompounds deposit with the germanium in the form of a white iilm ofnon-metallic appearance which separates out with the steel gray ger- 2manium hlm. 'Ihe deposited germanium film flakes oli` quite easily.

In accordance with this invention it has been found that germanium canbe plated out satisfactorily from solutions in the form of a brightmetallic coating. This can be accomplished by dissolving germaniumtetrachloride in a suitable organic solvent.

An object of this invention is to provide a suitable bath from whichVgermanium can be deposited electrolytically in the form of a brightmetallic coating.

A further object of this invention is to provide an improved methodl ofdepositing substantial lms of germanium electrolytically.

rThese objects and other advantages can be attained by dissolvinggermanium tetrachloride in a suitable organic solvent and electroplatingthe germanium therefrom.

In the drawings which illustrate preferred embodiments of this inventionFigure l shows a schematic view of an electroplating bath containinggermanium tetrachloride dissolved in ethylene glycol.

Figure 2 is a schematic view of another embodiment of an electroplatingbath containing germanium tetrachloride dissolved in propylene glycol.

Figure 3 is a curve showing cathode current efficiency of a propyleneglycol bath as a function of concentration.

In the preferred embodiment of the bath illustrated in Figure l lthetank l0 contains a solution I2 of germanium tetrachloride in ethyleneglycol from which germanium is deposited upon a copper cathode Ill.l Theanode I6 used in this bath is of cast germanium. Agitation means isprovided therein by means of a motor driven propeller type agitator 20.

In the embodiment illustrated in Figure 2 of the drawings the tank 3Gcontains a solution 32 of germanium tetrachloride in propylene glycol.The germanium is being plated onto a nickel cathode 36. The anode 34 inthis case is made of graphite. A propeller type agitator is there shownfor producing the desired agitation in the solution during the platingoperation.

When germanium is plated from a solution of germanium tetrachlorideprepared in accordance with this invention thick Well adhering germaniumiilms can be obtained. These metallic lms are of silver mirror-likebrightness and are of a high degree of purity. The baths mayconveniently be operated at temperatures in the neighborhood of 5060 C.,at which temperature neither decomposition nor appreciable loss ofsolvent nor solute occurs.

sconce In those cases in which the germanium tetrachloride is dissolvedin ethylene glycol the electrodeposition is preferably carried out Withthe use of a germanium anode of the type as shown schematically inFigure l of the drawings. If the best type of coatings are to beobtained more freedom of action is possible in those cases in whichpropylene glycol is used as the solvent for germanium tetrachloride. Inthese cases very good coatings of germanium can be obtained withgraphite anodes as well as with germanium anodes. There is reason tobelieve that substances like 1,2 butanediol or 1,2 pentanediol mightserve as solvents for germanium tetrachloride in the preparation of aplating bath.

Although germanium tetrachloride is quite soluble in ethylene glycol ithas been found desirable to keep the bath concentration relatively lowif good plating is to be obtained. If the concentration of the mixtureis too high, for example, in the neighborhood of vol. percent germaniumtetrachloride a precipitate of white crystals is formed upon standing.However, to achieve good results the concentration should be kept above.5 vol. percent of germanium tetrachloride if the bath is to be operatedat a cathode current density of 0.2 amp/cm.2 and a temperature of about50 C. At .5 vol. percent and below the deposits obtained are dull, gray,and rough. When the concentration goes as low as .1 vol. percent blacksmutty deposits are obtained. The cathode current enciency andappearance of deposits as a function of concentration in the germaniumtetrachloride-ethylene glycol bath can best be shown by the followingtable:

5U 1111. volumes solution Were electrolyzed at a cathode current dens1tyof 0.2 amp./cm.2, a temperature of 50 C. and agitation. Ge anode, Cucathodes.

The influence of current density and temperature on deposition in thegermanium tetrachloride-ethylene glycol bath can best be seen from thefollowing table.

l Current Pmjed'fm e 'D ensity Observations (amp/cnr?) 0.05 Depositionfirst observed if cathode 21--26 is exposed to high agitation.

(l. l5 First bright film. 0. 24. Bright deposition. 0.07 Depositionfirst observed. 0.11. 53-55 0.15.

0.15A Bright deposition. 0.20

0.30. l 0.05. No deposit. 0.10 Deposition iirst observed.

Bright deposits but of coarser grain 7& 30 than at lower temperatures.(The 0 20 rate of solution of the stop-ou' used 0. becomes larger athigher temperatures; this indirect effect of temperature may also havebeen responsible.)

Plating Conditions: Conc.=3.8 vol. percent GeCh throughout The bath wasagitated.

From the above table it is apparent that bright deposits can readily beobtained at a current density from about 0.1 amp/cm.2 upwards. It hasbeen found that an increase in temperature causes a large increase incathode current efciency so that a bath operated at about isapproximately four times as eilicient as one operated at 50.

If bright silvery deposits are to be obtained it is recommended that thebath be agitated. However, the rate of agitation at the cathode has nosignicant effect upon cathode current eiliciency.

For best results it is important that the moisture content of the bathbe controlled for if the water content should reach 20 milligrams ofwater per cc. no appreciable germanium will be deposited. In workingwith this bath it has been found that silver-like deposits of germaniumcan be plated on cathodes of many different types including copper,nickel, aluminum nickel alloy, zirconium, and graphite, etc.Furthermore, the throwing power of the bath seems excellent.

It has been further found that when platinum is made anodic in this bathit dissolves. For these reasons it may prove useful for the codepositionof platinum and germanium. Although graphite may be used as the anode inthe ethylene glycol-germanium tetrachloride bath the desired brightcoating is not as readily obtained as when propylene glycol is used inplace of ethylene glycol. However, when the germanium anode is used andthe germanium concentration is adjusted below the point whereprecipitation of a White, crystalline substance occurs, bright silverymirror-like deposits can be obtained continuously from this bath.

While it is obvious that electrodeposition of germanium from germaniumtetrachloride-ethylene glycol baths represents a considerableimprovement over prior art methods in that the bath deposits uniformlybright metallic germanium which adheres well to the base metal and whichdoes not contain organic or inorganic impurities, and While the bath canbe operated at a temperature at which neither the chloride nor thesolvent volatizes to any appreciable extent, the bath does, however,offer several objectionable features in that the use of the germaniumanode makes it impossible to plate at constant bath concentration andthe high anode current einciency eventually leads to a precipitationwhich calls for diluting the bath to permit further operation. Thesedifliculties are overcome by substitution of propylene glycol forethylene glycol. When this is done no solid phase precipitates eitherwhen preparing solutions of high germanium content or upon electrolysisof the bath for long periods of time. This permits the use of higherconcentrations in the bath with an actual increase in cathode currentefliciency and, hence, of plating speeds.

Furthermore, the rate of water absorption from the atmosphere is smallerfor propylene glycol than for ethylene glycol thus making it easier tomaintain the efficiency of the prcpylene glycol bath.

In addition, it has been found that this bath may be operated withgraphite anodes without decreasing the brightness of deposits. Thispermits operation of the bath at essential constant germaniumconcentration.

It has been found that there is a definite trend toward an increase ofbrightness of the deposits gecoate maniuin tetrachloride in an alkylglycol whose hydroxyl groups are present in the 1,2 position.-

2. A bath suitable for the electrodeposition of germanium attemperatures below 90 C. comprising a .5 to 10 volume percent solutionof germanium tetrachloride in ethylene glycol.

3. A bath suitable for the electrodeposition of germanium attemperatures below 90 C. cornprising a .5 to 10 volume percent solutionof germanium tetrachloride in propylene glycol.

[Eiect of temperature, current density and concentration upon appearanceof deposits and cathode current eciency (Ge+++2e- Ge).

. Germanium tetrachloride-propylene glycol bath] 0.2 amp./cm.' 0.3amp/cm.2 0.4 amo/cin.2

Vol. rgglae Percent Cath. oath. oath.

G9014 Appearance of Deposit Current Appearance of Deposit CurentAppearance of Deposit Orrent (Percent) (Percent) (Percent) 3 a. metallicsilvery 0. 558 bright with white hue 0.571 considerable amt. ofstreaking 0. 524

over bright surface. 5 metallic silvery (brighter 0.621 bright withslight streak- 0. 685 mirrorlike minute streaking 0.773 50.2 C than a).ing over entire surface. over surface, some streaking near. 7 b.rletalli silvery (brighter 0. 654 very bright 0. 689 mirrorlike 0. 818

an a 3 bright whitish huein part. 0.750 bright with good amt. oi 0.660large amt. of streaking over 0.651

streaking over surface. entire surface. 5 brighter than b, but not 0.868 almost mlrrorlike with 0.773 almost mirrorlike with slight 0.87759.3 Q ...A mirrorlike. slght streaking near streaking near edges.

e ges. 7 very bright but not 0.713 mirrorlke 0.810 perfect mirror 0. 843

mirrorlike.

Moisture content of propylene glycol=0.2 mg. H2O/m1. Cathode surfacespeed= 10 m./min. Graphite anode.

The cathode current eiciency of the propylene glycol bath at 60 and at acurrent density range of 0.2-0.4 ampere per square centimeter is bestseen in Figure 3 of the drawings.

t has been found that if the water concentration of the bath isincreased from .3 gram per liter to 1.3 grams per liter it will cause areduction in cathode current eiiciency of approximately 8.7%. Thecathode current efficiency of a propylene glycol bath is clearly shownin Figure 3 of the drawings. As there shown the maximum efficiency isobtained within the relatively narrow range of 2 to 8 Volume percent ofthe germanium tetrachloride. At concentrations above this range theeinciency of the bath falls off very markedly. The peak efficiency liesat about 5% with an optimum range of about 4 to 7%.

At optimum conditions for bright plating from the propylene glycol bath,7 vol. percent, 59 C., 0.4 amp/cm.2 using agitation and a graphiteanode, germanium deposits at the rate of 0.001l per three hours. It isnoteworthy that germanium can be deposited from propylene glycol bathsto a thickness of 0.005'l and still show the bright plated appearance.From the above it can be seen that while the propylene glycol baths givedeposits similar to those from ethylene glycol baths they show none ofthe diiiculties of operation inherent in the ethylene glycol bath.

t is of course to be expected that various changes, alterations andmodications will occur to those skilled in the art upon reading thepresent disclosure. It is to be understood, however, that themodiiications disclosed herein are described for illustrative purposesonly and are not intended to limit the scope of the appended claims.

What is claimed is:

1. A bath suitable for the electrodeposition of germanium attemperatures below 90 C. comprising a .5 to 10 Volume percent solutionof ger- 4. A bath suitable for the electrodeposition of germanium attemperatures below C'. comprising a .5 to 10 volume percent solution ofgermanium tetrachloride in 1,2 butanediol.

5. A bath suitable for the electrodeposition oi germanium attemperatures below 90 C'. comprising a .5 to 10 vol. percent solution ofgermanium tetrachloride in ethylene glycol.

5. The method of electroplating germanium from a .5 to l0 Volume percentsolution of germanium tetrachloride in ethylene glycol held at atemperature below 90 C. comprising passing a current from a germaniumanode to the metal base to be plated.

A method for producing a bright germanium plating on a metal base whichcomprises passing a current of .2 to .4 amp/cin.2 from a germanium anodeto the metal base to he plated through a bath formed essentially from a.5 to '7 vol. percent of germanium tetrachloride in ethylene glycolwhile said bath is held at a temperature below 90 C.

8. A method for plating germanium from a .5 to 10 volume percentsolution of germanium tetrachloride in propylene glycol which comprisespassing a current from a graphite anode to the metal base to be platedwhile said solution is held at a temperature below 90o C.

9. A method for producing a bright germanium plate on a base metal whichcomprises passing a current of 0.4 amp/cm.2 from a graphite anode to themetal base to be plated through a bath formed essentially of a 7 Vol.percent solution of germanium tetrachloride in propylene glycol whilesaid solution is held at a temperature below 90 C.

10. A method for plating germanium from a .5 to 10 Volume percentsolution of germanium tetrachloride in 1,2 butanediol which comprisespassing a current from a graphite anode to the metal base to be platedwhile said solution is held at a temperature below 90 C.

1l. A bath suitable for the deposition of germanium at temperaturesbelow 90 C. comprising 3-8 vol. percent of germanium tetrachloride inpropylene glycol.

12. A method of electroplating germanium from a .5 to 10 volume percentsolution of germanium tetrachloride in propylene glycol comprisingpassing a current from a germanium anode to the metal base to be platedWhile said solution is held at a temperature below 90 C.

13. A method for producing a bright germanium plating on a metal basewhich comprises passing a current of .2 to .fl amp/cm.2 from a germaniumanode to the metal base to be plated 8 percent of germaniumtetrachloride in propylen'e glycol while maintaining said bath at atemperature below 90 C.

14. A method for producing bright germanium plating on a base metalwhich comprises passing a current of 0.4 amp/cm.2 from a germanium anodeto the metal base to be plated through a bath formed essentially of a 7vol. percent solution of germanium tetrachloride in propylene glycol,said bath being operated at a temperature below 90 C.

References Cited in the file 0f this patent Trans. of theElectrochemical Society, vol. 95,

through a bath formed essentially of 3 to 8 vol. 15 1949, pages 80 and88-92 inclusive.

1. A BATH SUITABLE FOR THE ELECTRODEPOSITION OF GERMANIUM ATTEMPERATURES BELOW 90* C. COMPRISING TETRACHLORIDE IN AN ALKYL GLYCOLWHOSE HYDROXYL GROUPS ARE PRESENT IN THE 1,2 POSITION.